If you get an additional letter, then it seems the file without any suffix is already present in the directory. That would also explain the (2) as that is a common way to denote a second version of a file.
In the file listing you submitted I didn't see it, but I'm sure it must be there. Perhaps you have made it hidden somehow?
Here's what I suggest you do: open the folder in Finder (you're on a mac, right?), create a new folder (can be anywhere, even a subfolder), move the (2) files there. Open the new folder, rename the files there so they do not have the (2) part any longer (in the Finder: select a file hit enter and remove the (2) from the name), move them back to the original folder, I'm quite certain you will get a message that the files are already there. If not, problem should be solved too as all files will now be as if they came out of the camera.
Now you should be able to perform your rename in PM as required.
HTH,
Hayo
P.S. Rename that Mills_200988_A back to the original DSC_ file name before continuing too.